What is useful information today may become 'lore' in a few years so
it would be good to add additional data so that the reader (and
documentation maintainer) knows the vintage of the information.

That's a good point. Do you think it would be OK to put such information
only in Texinfo comments for the moment, and then, as a second and later
step, devise a way to report it in the final manual too? This second step
wouldn't be trivial IMHO, since we would need to present such version
information in a way that is at the same time clear, non-obtrusive and
complete (mabe we could take a look at how Gnulib does this?). Finally,
as a third step, we might try to revisit the existing examples of bugs
and portability pitfalls, and try to pin-point them to a precise version
of the system/tools used (in case this version is not already reported).

Perhaps specifying the OS release version the issue was noticed on
(not all-inclusive) or the date that the issue was noticed would be
sufficient? For example "FreeBSD 8.2" rather than "FreeBSD"? or
"FreeBSD (2011)" rather than "FreeBSD"? The drawback of this might be
that someone may assume that the issues are particular to that release
only.

Hardly anyone reads the raw Texinfo so comments in the Texinfo do not
seem very useful for the common reader. If the reader knows that the
current version of FreeBSD is 12.8, then he might want to check to see
if claims made about 8.2 (totally defunct by then) are still
applicable. Comments in the raw Texinfo from the authors regarding
versions when the issue was checked (or rechecked) would be useful.